Timers are used so that program executes things all the time, right ?
Get player input, simulate physics, render and stuff.
I dont know how can i "render" the bitmap with WM_PAINT when WM_TIMER is called.

It sounds like what you want is a game loop. (Though if you're writing a game... using WinAPI is not what I'd recommend... see below).

Your program already executes things "all the time". The problem is you are handing off control to the "GetMessage" function which means you will only be running when there's a message. To get a proper game loop... do not use GetMessage. Instead use the non-blocking PeekMessage:

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// in WinMain after all your setup is donewhile( game_is_running )
{
// process all incoming messageswhile( PeekMessage( &msg, wnd, 0, 0, PM_REMOVE ) )
{
TranslateMessage( &msg );
DispatchMessage( &msg );
}
// the above loop will happen virtually instantaneously... meaning you can put game// logic and other stuff here
doGameLogic();
// at this point, you can draw to the screen ( you don't need to use WM_PAINT for that,// you can just use GetDC()/ReleaseDC() )// but you also do not want the game to run too fast. So you'll need to regulate the// framerate to run at 60 FPS (or whatever framerate you want). This can be done by// keeping an eye on the time (GetTickCount()) and Sleep()ing until 1/60th of a second// has passed.
waitForNextFrame();
}

This is how games are typically done.

But as I mentioned before... using WinAPI is not my first recommendation for this. I strongly suggest you get a lib designed for this... like SFML. For a few reasons:

1) SFML is crossplatform
2) SFML is much, much, much easier
3) SFML gets much better performance
4) SFML can do a lot more out of the box (loading pngs, alpha blending, transparency, flipping, rotating, color tinting, audio output, gamepad input, etc)
5) SFML can vsync and/or has built in framerate regulation, so you can just say setFrameLimit(60); and it will automatically limit your program to run at 60 FPS without you having to worry about the details.